Members of several plant families synthesize large amount of predominantly medium-chain (C8-C14) triacylglycerols in specialized storage tissues, some of which are harvested for production of important dietary or industrial medium-chain fatty acids (F. D. Gunstone, The Lipid Handbook (Chapman & Hall, New York, 1986) pp. 55-112). Laurate (C12:0), for example, is currently extracted from seeds of tropical trees at a rate approaching one million tons annually (Battey, et al., Tibtech (1989) 71:122-125).
The mechanism by which the ubiquitous long-chain fatty acid synthesis is switched to specialized medium-chain production has been the subject of speculation for many years (Harwood, Ann. Rev. Plant Physiol. Plant Mol. Biology (1988) 39:101-138). Recently, Pollard, et al., (Arch. of Biochem. and Biophys. (1991) 284:1-7) identified a medium-chain acyl-ACP thioesterase activity in developing oilseeds of California bay, Umbellularia californica. This activity appears only when the developing cotyledons become committed to the near-exclusive production of triglycerides with lauroyl (12:0) and caproyl (10:0) fatty acids. This work presented the first evidence for a mechanism for medium-chain fatty acid synthesis in plants: During elongation the fatty acids remain esterified to acyl-carrier protein (ACP). If the thioester is hydrolized prematurely, elongation is terminated by release of the medium-chain fatty acid. The Bay thioesterase was subsequently purified by Davies et al., (Arch. Biochem. Biophys. (1991) 290:37-45) which allowed the cloning of a corresponding cDNA which has been used to obtain related clones and to modify the triglyceride composition of plants (WO 91/16421 and WO 92/20236).